Tomography is a process that uses penetrating waves to create an image. Muon tomography is a technique that uses muons to generate three-dimensional images of volumes based upon scattering of the muons as they pass muon drift tubes of a muon drift tube array.
A muon drift tube is basically a long cylindrical aluminum tube filled with a specific combination of gases, for example, Ar:CO2 (93:7) or Ar:C2H6:CF4 (49:7:44), designed to be sensitive to the passage of muons. The tube includes a thin wire, under tension, running through its center. In use, the wire acts as an anode and the aluminum tube acts as a cathode such that, when a high voltage is transmitted across the wire, electrons generated from a reaction between the muon and gas particles produce ions that will precipitate towards the wire. From this reaction, the drift time of the ionization electrons in the gas can be used to approximate the angle and position of the muon crossing the tube using a technique commonly referred to as “multiple-coulomb scattering.”
There are multiple factors that affect the reliability, and consequently the performance of a drift tube. For example, temperature changes will cause the aluminum drift tube to expand and contract. The expansion and contraction, in turn, causes a change in the volume of the drift tube. As a result, an increase in temperature at constant pressure will result in both a lower gas density and higher drift velocity. Conversely, a decrease in temperature at constant pressure will result in both a higher gas density and lower drift velocity.
Muon drift tubes commonly have two specific causes of performance degradation and, ultimately, their failure—gas leakage and contamination.
In use, muon drift tubes are directly subject to temperature fluctuations. Moreover, it is not possible to create a perfect seal among the drift tube components that can remain intact through the in-use temperature fluctuations experienced by the muon drift tubes over a long period of time. As a result, gas leakage from a muon drift tube, over time, is a problem because it results in an inconsistency in the volume of gas within the muon drift tube over time. Moreover, it is a greater problem when a mixture of gasses is used, because the gases will have different leakage rates due to their different densities, causing a change in the mixture itself.
Another source of degradation and failure is contamination of the gas mixture by infiltration of contaminants, such as water, air and hydrogen. Contaminant infiltration affects, among other things, the electron drift time over time. Moreover, compensating for contamination is very difficult because it a variable that is often a function of the physical environment at the specific drift tube location.
To ensure that the muon drift tubes have sufficient life and reliability over time, the joint between the tubes and caps must be sealed, at manufacture, to a Helium gas leak rate of less than 1×10−8 bar l/s as measured by a mass spectrometer leak detector.
Meeting this requirement requires the use of multiple special component parts and incorporation of special, complex features into the components, and the manufacture further involves the use of complex, laser welding or crimping equipment that is expensive to acquire and maintain. Still further, the manufacture must be performed by well-trained/experienced operators/engineers. All of the foregoing leads to a high manufacturing cost.
Thus, there is a need in the art for a simpler, less expensive way to produce those muon drift tubes, that can nevertheless meet the current, stringent, hermeticity requirements.